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Full text of "Handbook Of Chemical Engineering - I"

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LEACHING AND DISSOLVING                            353
we see that there is a real attempt made to raise the solids selectively and let them percolate through the solution and to raise them again when they settle out. The solution elevated by the air lift will not be solids only but a pulp mass whose density above the average density depends upon the vigor with which the agitation is conducted.
There is also a violent agitation caused in the central tube in elevating the liquid so that the air consumed here is for agitation as well as raising the solids. It is found that there is a tendency for the coarser material which requires a longer treatment than the finer sands to collect in the agitator so that it received from one to four times the treatment that the more flocculent material obtains. Total power consumption is about 1.5 horsepower for a tank 20 by 10 ft.
Pachuca.—This type of agitator, similar to the Dorr in. what it attempts to accomplish, has in recent years been almost completely replaced by other types. It consists of a very high tank with a conical bottom. Reaching from the top of the tank to the center of this cone is a pipe to be used as an air lift as in the Dorr, the idea being that as the particles sink they strike this conical bottom and slide
FIG. 6.—Dorr multi-deck washing classifier.
to the apex of the cone where they encounter the air lift. Here they are raised to the top and are distributed over the surface of the solvent as in the Dorr and again sink to the bottom.
The difficulty with this agitator is that the solids build up on the sides of the cone and then all slide down at once causing the air lift to become clogged or at best to work very irregularly. In order to increase the slope of the cone, the tanks are very high to get large capacity causing "great expense in erecting and difficulty in placing under cover.
Dorr Classifier.—The Dorr classifier is being used in a number of operations for washing granular material free from value-bearing solutions or even directly for dissolving easily soluble materials. The multiple-deck Dorr classifier, especially designed for this work, is made up of a number of inclined settling boxes as shown in Fig. 6 in each of which is a set of rakes operated by one mechanism which move the solids settling out along the incline to the next compartment. The material to be washed is fed into the first compartment while the wash water is fed into the third and flows by gravity through pipes on the side connecting this compartment to the second and then the first where it is discharged. The height of the solution in the compartments is such that a draining period is allowed for every wash.